1. Field of the Invention
The present invention relates generally to the field of optical fiber signal transmission. More particularly, it relates to apparatus and methods for the remote measurement of physical parameters using fiber optic elements including a remote optical circulator in a system of optical fiber cables and optical fiber sensors.
2. Description of Related Art
As oil and gas reserves have been increasingly consumed over the years, the extraction of these hydrocarbons has become more difficult. The resultant development and exploitation of remote oil and gas resources in increasingly difficult operating environments such as deep water have given rise to numerous new technological challenges. Notably, there is an increased need for reserves and wells to be more widely monitored, especially for those hydrocarbon reserves lying deep below the ocean seabed.
Recent developments in fiber optic sensing technology, such as optical fiber sensors and optical fiber cables to link the sensor to the measurement instrumentation, have resulted in new and improved alternatives to the conventional electronic systems used in downhole production and reservoir monitoring. Optical fiber technology offers numerous advantages over past electronic monitoring systems, as they are able to withstand high pressures and temperatures. Furthermore, optical fiber systems and optical fiber sensors are typically of a structure and diameter similar to the optical fiber cable itself, allowing for easy incorporation into the downhole system.
In-well fiber optic systems measure such parameters as temperature, pressure, flow rate, fluid phase fraction, and seismic response, among other things. In such systems, light is sent along a single optical pathway (e.g., an optical fiber), and is reflected from the optical sensors such a Bragg grating sensors coupled to or incorporated with the pathway. The reflected light, indicative of the measured parameters, is sent back along the optical pathway for analysis. Such optical systems combine a high level of reliability, accuracy, resolution, and stability, and permit the multiplexing of several sensors along the optical pathway, thus enabling complex and multilateral wells to be fully instrumented with a single optical array. Through the use of such advanced fiber optic systems, real-time downhole data can be retrieved and analyzed to greatly improve production management and reservoir recovery. The value of such real-time, downhole monitoring systems offer the promise of achieving high levels of performance with low costs.
However, the use of fiber optic systems in such environments has resulted in several significant problems that have limited their use to date. Optical scattering phenomenon, such as Rayleigh backscatter in reflective single-fiber optic sensor transmission line systems, can limit the achievable deployment distances. Similarly, Mie (scattering of visible light wavelengths by spherical particles), Brillouin (scattering due to the interaction of laser light with sound waves) and Raman (scattering of laser light as it passes through a transparent medium) scattering phenomena further limit the distance over which optical sensing systems can be employed due to the elevated signal-to-noise ratio they cause. Other optical scattering noise such as Freznel (reverse propagating) reflections due to the connectors or couplers used in optical fiber technology can further contribute to high signal-to-noise ratio. These intrinsic (Raman, Mie, Brillouin, Rayleigh) and extrinsic (Freznel) effects add to the limit of achievable deployment distances in optical fiber monitoring technology, and suggest that expensive lower-loss fiber optic splices, instead of connectors or couplers, should be used when connecting components together along the array.
While there have been numerous patents and publications describing methods for measuring physical parameters using fiber optic systems, few address the issue of backscatter noise and limited monitoring distance. See, e.g., U.S. Pat. No. 5,361,313 to O'Keefe (describing a fiber optic sensor capable of detecting multiple parameters in remote locations using a combination of polarized light and multi-mode fiber optics); U.S. Pat. No. 5,582,064 to Kluth (describing a remotely deployable pressure sensor with a pressure communicating means operable by remote control); U.S. Pat. No. 6,006,832 to Tubel (describing a method and system for monitoring a formation surrounding a borehole in which a remote central control center communicates information with remote well platforms via telephone or wirelessly via satellite). Other approaches to this problem involve amplification of the reflected signals at the wellhead. However, these approaches suffer from several limitations. First, electronic equipment may be unable to withstand the harsh conditions of a downhole system, and in the event of a failure or breakdown would be very difficult and expensive to retrieve. Similarly, while placing an amplifier at the wellhead would increase the amplitude of the reflected signal, the associated noise of optical backscattering phenomena would increase as well.
What is needed is an optical fiber monitoring system that will allow for the remote measurement of physical parameters over significant distances without being limited by optical scattering phenomena.